Previous computers have been provided having a capability of suspending the CPU clock for some period of time in order to save power, such as during those periods in which the computer is not being used. This so-called "sleep" mode saves some amount of power. For example, in a device which uses about 2.67 watts of power during normal operation, sleep mode will permit such a unit to use only about 2.02 watts of power. Such sleep mode is sometimes useful in connection with lap-top or other battery-powered, or limited power-source, computers.
It would be further useful to reduce power to an even lower level. Of course, power to the computer can simply be turned off for a period. However, in this case, data and instructions which are in dynamic memory will be lost. It is possible to store data and programs in non-dynamic memory, such as static random access memory (SRAM). SRAM also has less stringent voltage requirements than DRAM, which typically must have a regulated 5-volt power supply with little deviation therefrom. This is an additional reason why SRAM is conventionally used as non-volatile memory. However, these memory devices are expensive and, perhaps of more consequence to lap-top applications, require greater volume and have greater mass than corresponding dynamic memories.